Metadata Viewer: API to return dict, not list

[pllim]

Description

This pull request is to address the request, "File metadata extracted from the API is hard to parse. Maybe a dictionary is better?"

I avoided changing metadata itself because that would mean messing with the front-end stuff that already works. Also might be subtly confusing because the same attribute now returns something different. Maybe best to have a clean break using a different attribute name.

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone. Bugfix milestone also needs an accompanying backport label.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)? 🐱

[pllim]

Without this, I get things in Cubeviz that is not from original header like:

• original_spectral_axis_unit (a Quantity)
• original_wcs (WCS object)

Perhaps these should have been hidden with leading underscore but they snuck through so here we are.

[pllim]

Weird thing is I don't know where these are being injected. A simple grep in the code base gave me no match.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.80%. Comparing base (f715ffe) to head (44a102e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3292   +/-   ##
=======================================
  Coverage   88.80%   88.80%           
=======================================
  Files         125      125           
  Lines       19009    19012    +3     
=======================================
+ Hits        16881    16884    +3     
  Misses       2128     2128           

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